This invention relates to an improved castable high explosive, particularly to a castable explosive composition containing explosives which melt and/or decompose at temperatures higher than about 90.degree. C.
Castable explosives are classified either as melt/cast or as plastic bonded. Melt/cast systems require the melting of the explosive, such as TNT (mp 81.degree. C.), and casting into a munition. Plastic bonded systems involve a mixture of one or more explosives with a polymeric binder, casting into a munition, and curing of the binder.
Explosive compositions based on TNT constitute an important class of military explosives. Examples of such explosive compositions include Composition B (RDX-cyclotrimethylenetrinitramine and TNT-2,4,6-trinitrotoluene), Tritonal (TNT and aluminum powder), cyclotols (RDX and TNT in ratios of 75:25 to 60:40), Amatol (ammonium nitrate and TNT), Kalatol (potassium nitrate and TNT), Baratol (barium nitrate and TNT, Pentolite (PETN-pentaerythritol tetranitrate and TNT) and Baronal (barium nitrate, aluminum and TNT). Such compositions are generally prepared by stirring the solid powdered components, which include such materials as RDX, HMX (cyclotetramethylenetetranitramine), PETN, NQ (nitroguanidine), ammonium nitrate, potassium nitrate, barium nitrate, lead nitrate, potassium perchlorate, ammonium perchlorate, aluminum powder, and the like, as well as mixtures thereof, which are essentially insoluble in TNT, with molten TNT until a homogeneous dispersion or slurry is obtained. The molten slurry is loaded into shells or other ordnance items by pouring into the cavity and allowing the melt to cool and solidify. Such slurries, which in concentrated cases are very viscous and difficult to pour, exhibit undesirable segregation and settling of the solids during the period required for cooling and solidification, thereby producing casts of non-uniform composition, which adversely affects the explosive and ballistic characteristics of the item. In the past, small amounts of additives, such as finely divided silica, silicone resins, nitrocellulose and other cellulosic resins, have been added to improve the suspension of the solid components in the TNT slurry.
TNT-based explosive compositions have a tendency to exude low melting impurities such as dinitrotoluene and mononitrotoluene, as well as unsymmetrical isomers of trinitrotoluene. It is known that high molecular weight nitrocellulose has the propensity to accept these low melting impurities of the composition. As a result of substantially high uptake in this regard, relatively small amounts of the nitrocellulose are necessary to prevent exudation of the cast. Thus, the viscosity of the melt is not appreciably altered by the addition of such cellulosic additive and the pourability of the composition is maintained. It has also been found that by such addition of cellulosic material, an explosive cast is obtained having greater mechanical strength and enhanced heat resistance.
In addition to exudation, TNT-based explosive compositions tend to crack upon temperature cycling, decompose autocatalytically upon exposure to fire and explode or detonate when in a bomb or warhead configuration, and further tend to detonate on sudden impact. These drawbacks have been partially corrected in the past by using wax type coatings as desensitizers and process aids. Beeswax was used initially at a level of about 1 wt. percent to desensitize Composition B. Shortages of beeswax led to a search for substitutes which concentrated on paraffin wax, microcrystalline waxes and synthetic waxes. Such wax coatings are deposited irregularly as agglomerates onto the explosive composition. As such, wax coatings tend to separate from the explosive composition surface upon exposure to heat and create potential cook-off conditions.
The waxes employed heretofore are generally not soluble in TNT. In order to form an emulsion of the explosive and wax, emulsifying aids are often added, as for example, lecithin as an emulsifier. Dispersions of 5% or greater of paraffin wax in TNT-based explosives are not truly stable emulsions.
As noted previously, plastic bonded systems involve a mixture of one or more explosives with a polymeric binder. Binders used heretofore include polyester, polyurethane, polystyrene, fluorinated polymers and various synthetic rubbers. Plastic bonded systems require thorough mixing of the ingredients with special emphasis on the quantity and mixing of the curing agent or catalyst. Once cast into a munition, a cure time of several days, at an elevated temperature, is required to ensure complete cure of the binder.
Plastic bonded systems have an advantage over conventional melt/cast systems in that virtually any explosive can be cast into a munition. In particular, explosives can be cast into munitions independent of the melt temperature of the explosive(s), while conventional melt/cast systems depend on at least one explosive ingredient being meltable at a safe processing temperature. The phrase "safe processing temperature" generally means about 121.degree. C. or lower which corresponds to the temperature of steam at 15 psig.
Melt/cast systems have an advantage over plastic bonded systems of higher processing and loading capacity. The melt/mix time is relatively much shorter, process control is not as critical, and no curing time is required. Melt/cast systems have a further advantage if unloading of the munition is ever required in that the cast explosive can be melted using a low pressure steam.
What is desired is an explosive composition which has the advantages of both the melt/cast and plastic bonded systems. The desired composition alloys melt/cast processing without dependence on the melt temperature of the explosive ingredients.
It is an object of the present invention to provide a novel explosive composition.
It is another object of the present invention to provide an improved method for melt/cast loading of munitions.
Other objects, aspects and advantages of the present invention will be apparent to those skilled in the art.